Lithographic techniques include the steps of forming a resist film made of a resist material on a substrate, selectively exposing the resist film to radiation such as light or an electron beam through a mask with a predetermined pattern formed thereon, and developing the resist film to form a resist pattern having a predetermined shape on the resist film.
A resist material having characteristics which cause the exposed portions to become soluble in a developing solution is referred to as a positive resist material, whereas, a resist material having characteristics which cause the exposed portions to become insoluble in a developing solution is referred to as a negative resist material.
In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have lead to rapid progress in the field of miniaturization.
Typically, these miniaturization techniques involve shortening the wavelength of the exposure light source. Specifically, ultraviolet radiation such as g-line and i-line radiation has conventionally been used, but nowadays KrF excimer lasers and ArF excimer lasers have begun to be introduced in the mass production of semiconductor elements. Also, radiation having a shorter wavelength than that of these excimer lasers, for example, F2 excimer lasers, electron beams, EUV (extreme ultraviolet), and X-rays are also being examined.
It is required that the resist material has sensitivity to these exposure light sources, and lithographic characteristics such as resolution capable of reproducing a pattern of minute dimensions. As the resist material which satisfies these requirements, a chemically amplified photoresist containing a base resin which exhibits changeable alkali solubility under the action of an acid, and an acid generator component which generates an acid upon exposure is used. For example, a positive chemically amplified photoresist contains, as the base resin, a resin which exhibits increased alkali solubility under the action of an acid and an acid generator, and when an acid is generated from the acid generator upon exposure in the formation of a resist pattern, the exposed portions are changed to an alkali-soluble state.
As the base resin of the chemically amplified photoresist, polyhydroxystyrene (PHS) having high transparency to a KrF excimer laser (248 nm) and a resin (PHS-based resin) containing a hydroxyl group protected with an acid dissociable, dissolution inhibiting group have conventionally been used. However, the PHS-based resin contains an aromatic ring such as a benzene ring and is therefore insufficient in transparency to light having a wavelength shorter than 248 nm, for example, light having a wavelength of 193 nm. Therefore, a chemically amplified photoresist containing the PHS-based resin as a base resin component has a drawback such as low resolution in a process using light having a wavelength of 193 nm.
Therefore, as the base resin used in ArF excimer laser lithography, a resin (acrylic resin) containing a structural unit derived from a (meth)acrylate ester in the main chain (see, for example, Patent Document 1) is mainly used because it is excellent in transparency at around 193 nm.
In view of a pattern forming method, in addition to a countermeasure for ultraminiaturization in view of a resist material, techniques for exceeding the limitation of resolution of the resist material have been studied and developed.
As one of these miniaturization techniques, for example, there is proposed a thermal flow process in which a resist pattern is formed by a conventional lithography technique and the resist pattern is subjected to a heat treatment thereby conducting miniaturization of the pattern size (see, for example, Patent Documents 2 and 3).
The thermal flow process is a process in which a resist pattern is once formed by a photolithography technique and the resist pattern is softened and allowed to flow in the space direction of the pattern by heating, thus decreasing the pattern size of a resist pattern, namely, the size (pore diameter of a hole pattern, space width of a line and space (L&S) pattern, etc.) of the portions on which no resist pattern is formed.
(Patent Document 1)
Japanese Unexamined Patent Application, First Publication No. 2003-241385
(Patent Document 2)
Japanese Unexamined Patent Application, First Publication No. 2000-188250
(Patent Document 3)
Japanese Unexamined Patent Application, First Publication No. 2000-356850